A modulation scheme used for radio communications such as a modern mobile phone and the like has a high-frequency utilization efficiency and a high peak-to-average power ratio (PAPR). In order to amplify a signal to which an amplitude modulation is applied by using an AB class amplifier that has been conventionally used in a radio communication field, it is necessary to use an amplifier operating with sufficient back-off to maintain a linearity. Generally, the required back-off value is at least approximately equal to a value of the PAPR. However, in the AB class amplifier, the maximum efficiency is obtained when it operates at the saturation point and the efficiency of the amplifier decreases with increasing the back-off value. Therefore, it is difficult to improve the power efficiency of the power amplifier for amplifying a high-frequency modulation signal having a high PAPR.
As a power amplifier for amplifying a modulation signal having a high PAPR with high efficiency, a polar modulation power amplifier is used. In the polar modulation power amplifier, the high-frequency modulation signal used for radio communication is generated from polar coordinate components of amplitude and phase. FIG. 9 shows an example of the polar modulation power amplifier (high-frequency power amplification circuit) disclosed as related art in Non-Patent Literature 1.
The circuit shown in FIG. 9 includes a high-frequency modulation signal input terminal 101, an amplitude signal input terminal 102, a power supply circuit 103, a high-frequency power amplifier 104, and a high-frequency modulation signal output terminal 105. The power supply circuit 103 further includes a linear amplifier 106, a subtractor 107, a current detection resistor 108, a hysteresis comparator 109, a switching amplifier 110, an inductor 111, and a power supply terminal 112.
A harmonic modulation signal that is amplitude-modulated or phase-modulated is input to the high-frequency modulation signal input terminal 101 and this harmonic modulation signal is transmitted to the high-frequency power amplifier 104. An amplitude signal in the harmonic modulation signal input through the high-frequency modulation signal input terminal 101 is input to the amplitude signal input terminal 102. The signal input through the amplitude signal input terminal 102 is highly efficiently amplified by the power supply circuit 103 and is supplied from the power supply terminal 112 as a power supply of the high-frequency power amplifier 104. The high-frequency power amplifier 104 amplifies the signal input through the high-frequency modulation signal input terminal 101 and outputs the amplified signal to the high-frequency modulation signal output terminal 105.
The power supply circuit 103 has a configuration in which both the switching amplifier 110 and the linear amplifier 106 are arranged so as to amplify the input signal in high efficiency and with low distortion. The amplitude signal input through the amplitude signal input terminal 102 is input to the linear amplifier 106. The output impedance of the linear amplifier 106 is low. The linear amplifier 106 linearly amplifies the input signal and outputs the amplified signal. The signal output by the linear amplifier 106 is transmitted to the power supply terminal 112 through the current detection resistor 108.
The subtractor 107 is connected to both ends of the current detection resistor 108 and outputs a value obtained by subtracting a voltage of the power supply terminal 112 from a voltage of the output signal of the linear amplifier 106. Here, because the input impedance of the subtractor 107 is high, the subtractor 107 does not consume a large amount of electric power supplied to the power supply terminal 112 and the output signal of the linear amplifier 106. Further, because the impedance of the current detection resistor 108 is set to low, the voltage applied to both ends of the current detection resistor 108 is negligibly small compared to the voltage applied to the power supply terminal 112.
The subtractor 107 outputs the output signal, which is a subtraction result, to the hysteresis comparator 109. The hysteresis comparator 109 makes a sign determination of the input signal and outputs the result of the determination to the switching amplifier 110. However, the hysteresis comparator 109 has a function to hold the latest output state and has a hysteresis width (V_hys), if the latest output state is “low”, the output state changes to “high” when the input signal level becomes equal to or greater than V_hys/2 and if the latest output state is “high”, the output state changes to “low” when the input signal level becomes equal to or smaller than −V_hys/2.
The switching amplifier 110 amplifies the signal input through the hysteresis comparator 109 and outputs the amplified signal to the power supply terminal 112 via the inductor 111. In this case, the current supplied from the switching amplifier 110 via the inductor 111 and the current supplied from the linear amplifier 106 via the current detection resistor 108 are combined and the power is supplied from the power supply terminal 112.
The above-mentioned power supply circuit 103 has two advantages: high linearity of the linear amplifier 106 and high efficiency of the switching amplifier 110. This is because in the power supply circuit 103, the output voltage is determined by the linear amplifier 106 having low output impedance and most of the output current is supplied by the switching amplifier 110 with high efficiency. The current output through the power supply terminal 112 is a sum of the output current of the linear amplifier 106 and the output current of the switching amplifier 110. A potential of the power supply terminal 112 is determined by the linear amplifier 106 having low output impedance. In order to maintain the electric potential of the power supply terminal 112 to a target value, the current is supplied by the linear amplifier 106. The output current of the linear amplifier 106 is detected by using the current detection resistor 108 and the hysteresis comparator 109 and the current supplied by the switching amplifier 110 is adjusted so that the output current of the linear amplifier 106 is prevented from becoming excessive.
By using the above-mentioned method, most of the current output through the power supply terminal 112 is supplied by the switching amplifier 110 and the output current of the linear amplifier 106 can be used only for correction of an error component of the switching amplifier 110.